Pearl pigments are used widely as one of color materials in various fields such as inks, coatings, plastics and cosmetics etc. These pearl pigments are prepared by covering a flaky substrate thereon with a transparent or semitransparent metal oxide having high refractive index so as to emit the interference color from mutual interfering action between reflection of incident light from the interface of a flaky substrate/covering layer and reflection of incident light from the surface of the covering layer. Because these pearl pigments are composed essentially of inorganic compounds, they have high polarity and there appears the problem of poor affinity to organic media.
Accordingly, in order to improve their affinity to organic media and anti-yellowing property, pearl pigments treated with various methods have been proposed (e.g., JP-A 63-130673, JP-A 1-292067, JA-A 4-296371, JA-A 5-171058, JA-A 6-16964 and JP-A 9-48930). However, these treatments are concerned about offering only the improvement of the affinity of the flaky pearl pigment to organic media, the orientation of the flaky pearl pigment readily becomes random in the medium and the flaky pearl pigments is just uniformly dispersed in the medium, so the original characteristics resulting from the shape of the flaky pearl pigment cannot be sufficiently demonstrated. That is, it is impossible to arrange the flaky particle plane of the flaky pearl pigment to parallel to a printed surface or a coated surface (hereinafter, this arrangement is referred to as “plane orientation”) in order to achieve the maximal original effect of the coloristic property of the pigment. By improving or devising mechanical methods such as the method of printing, coating or plastics moulding etc, the probability of plane orientation is increased to a certain degree but this is still insufficient to achieve the adequate effect. Accordingly, there remains a great need in the market for higher demonstration of characteristics originating in the shape of the current pearl pigment (hereinafter, the improvement of coloration etc. by plane orientation is referred to as “leafing effect”).
The technical problem for this need requires a technique balancing delicately both the effect of affinity to a medium and the effect of floating on the surface of the medium, to prevent flaky pearl pigments from being mutually aggregated in an organic medium, and simultaneously to float the pigments to be as parallel as possible on the surface of the medium without dispersing them into the inside of the medium.
On the other hand, JP-A 9-48707 discloses that the flaky pearl pigment coated thereon with spherical fine particles of acrylic resin belonging to fine particles of ethylenically unsaturated carboxylic acid-type resin achieves natural flesh color finishing due to diffused reflection of reflected light from the surface of the pearl pigment when used as makeup cosmetics.
In view of the description therein “Paying attention to the excellent optical characteristics of the pearl pigment, the intensity of uniformed and, diffused reflection light can be arbitrarily controlled by covering its strong glossiness in the specular direction with spherical fine powder of acrylic resin . . . ”, this prior art is directed to the improvement (halation effect) of optical characteristics for mere use in cosmetics, and such a pearl pigment cannot be expected to exhibit the leafing effect. In fact, it is disclosed therein that the ratio by weight of the spherical fine particles of acrylic resin to the flaky pearl pigment shall be in the range of 60/40 to 40/60, and thus its surface is coated with a very large amount of fine particles of acrylic resin, and it is absolutely impossible to exhibit the leafing effect.
Further, in view of the process for producing the same, it is described therein that a pearl pigment and spherical fine powder of acrylic res in are mixed at a certain ratio, formed into slurry in aqueous alcohol and spray-dried at 70 to 100° C. thereby permitting the acrylic resin to adhere to the surf ace of the pigment to give the desired product, but according to this process, the spherical fine powder of acrylic resin may be dissolved and deformed owing to use of the alcohol, and thus it is not necessarily possible to permit the spherical fine powder of acrylic resin, while maintaining the spherical shape, to adhere to the surface of the pigment. Further, because the coating method is merely due to the method of spray-drying, its adsorption is based on physical adsorption and the adhesion force is weak, thus causing the acrylic resin powder to be exfoliated in some cases depending on the conditions to be done later (conditions for mixing in other media).